A Tale of Two Galaxies: Light and Mass in NGC 891 and NGC 7814

University of Groningen

A tale of two galaxies Fraternali, F.; Sancisi, R.; Kamphuis, P.

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DOI: 10.1051/0004-6361/201116634

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Citation for published version (APA): Fraternali, F., Sancisi, R., & Kamphuis, P. (2011). A tale of two galaxies: Light and mass in NGC 891 and NGC 7814. Astronomy & astrophysics, 531, [A64]. https://doi.org/10.1051/0004-6361/201116634

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Download date: 01-10-2021 A&A 531, A64 (2011) Astronomy DOI: 10.1051/0004-6361/201116634 & c ESO 2011 Astrophysics

A tale of two galaxies: light and mass in NGC 891 and NGC 7814

F. Fraternali1, R. Sancisi2,3, and P. Kamphuis2,4

1 Astronomy Department, University of Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: [email protected] 2 Kapteyn Astronomical Institute, Postbus 800, 9700 AV, Groningen, The Netherlands 3 INAF – Astronomical Observatory of Bologna, via Ranzani 1, 40127 Bologna, Italy 4 Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany

Received 2 February 2011 / Accepted 10 May 2011

ABSTRACT

The two edge-on galaxies NGC 891 and NGC 7814 are representative of two extreme morphologies: the former is disk-dominated while the latter is almost entirely bulge-dominated. It has been argued that since the two galaxies, which are optically so different, have similar rotation curves their total mass distributions cannot be related in any way to the light distributions. This would lead to the conclusion that dark matter is the dominating component of the mass. We have derived new rotation curves from recent, high- sensitivity H I observations and have found that the shapes of the rotation curves are significantly different for the two galaxies. They indicate that in NGC 7814 the mass is more concentrated to the centre as compared to NGC 891. This reflects the distribution of light which is more centrally concentrated in NGC 7814 than in NGC 891. Mass and light do seem to be closely related. This is confirmed by the analysis of the rotation curves in mass components: solutions close to the maximum light (bulge + disk) do provide excellent fits. In NGC 891 bulge and disk can explain the rotation curve without any need for dark matter out to ∼15 kpc. In NGC 7814 the bulge dominates in the inner parts; further out the rotation curve is well reproduced by a maximum disk but its M/L ratio is excessively high. A substantial dark matter contribution, closely coupled to the luminous component, seems, therefore, necessary. Key words. galaxies: kinematics and dynamics – galaxies: structure – galaxies: individual: NGC 891 – galaxies: individual: NGC 7814

1. Introduction not known inside 1 arcmin (3−4 kpc) because H I emission was not detected there. The conclusion was that “if the distribution The distribution and relative importance of luminous and dark of luminous matter were in any way related to the total mass dis- matter in galaxies are still a matter of debate. The distribution of tribution it would not be possible for the disk-dominated and the mass in a spiral galaxy is inferred from its rotation curve. In the bulge-dominated galaxies to have such similar rotation curves”. past three decades, rotation curves have been derived for a num- If the two galaxies had indeed rotation curves of such similar ber of spirals of various masses and morphological types. For the shape this would be contrary to the rule that in spiral galaxies interpretation of the rotation curves in terms of mass distribution there is a close correlation between light distribution and rotation use has been made of multicolour photometry. The conclusion curve shape (Sancisi 2004). The contrast would also exist with has been that in the outer parts of galaxies there is a significant non-Newtonian theories of gravity which would predict grossly discrepancy between the observed curve and the curve predicted different rotation curves (van der Kruit 1995). There is little from the photometric and gas profiles. Such discrepancy is usu- doubt that the light distributions of NGC 891 and NGC 7814 ally interpreted as evidence for the presence of dark halos around are very different. But, do their rotation curves have indeed es- spiral galaxies. For the bright inner parts of the disk, inside R25, sentially identical shapes? there is no such consensus. There is a range of possibilities from The rotation curves used by Van der Kruit for NGC 891 “maximum disks” (Kalnajs 1983; van Albada & Sancisi 1986; (Sancisi & Allen 1979) and for NGC 7814 (van der Kruit Kent 1986) with constant values of the M/L ratio (implying that & Searle 1982) were obtained from H I observations with the luminous matter dominates) to “minimum disks” with dark mat- Westerbork Synthesis Radio Telescope (WSRT). The rotation ter being the dominant component everywhere. curve of NGC 891 was obtained directly from the observations The debate on the relative distribution of luminous and dark using a velocity “envelope” method, that of NGC 7814 by con- matter in spiral galaxies is still open. The comparison between structing models. Both curves appear to be flat to a first approx- the two edge-on galaxies NGC 891 and NGC 7814 provides a imation, but the H I observations, especially for NGC 7814, had good illustration. These two spirals are representative of two ex- poor signal/noise ratios in the central regions and the curves had treme morphologies: a disk-dominated NGC 891 and an almost large uncertainties or could not be derived at all. entirely bulge-dominated NGC 7814 (see Fig. 1). Van der Kruit In recent years, new H I observations were obtained for (1983, 1987, 1995) pointed out that, in spite of this striking dif- these two galaxies with better sensitivity with the WSRT. From ference in their light distributions, these two galaxies have, be- these observations we derived new rotation curves. We used yond the central 3−4 kpc, “essentially identical” rotation curves. Spitzer 3.6 μm observations to derive the photometric param- These curves were measured in the H I 21-cm line but they were eters of the bulge and the disk components and perform mass Article published by EDP Sciences A64, page 1 of 9 A&A 531, A64 (2011)

Fig. 1. Left: multi-band (Y, J, K) image of NGC 891 obtained with the WIRCam at the Canada-France-Hawaii Telescope (CFHT). Right: multi- band (g, r, i) image of NGC 7814 from the Sloan Digital Sky Survey.

Table 1. Optical and H I parameters for the two galaxies. package and standard calibration was applied. The result is a data cube consisting of 160 channels with a spatial resolu- tion (FWHM)of54.5 × 12.9 and with a velocity spacing of Parameter NGC 891 NGC 7814 Ref. −1 Morphological type Sb/SBb Sab 1, 2, 3 4.12 km s . Centre (α J2000) 2h22m33.41s 0h03m14.89s ◦ ◦ The derivation of the rotation curves was done in two steps. (δ J2000) 42 20 56.9 16 08 43.5 A first estimate was obtained by taking the envelope on the high Distance (Mpc) 9.5 14.6 1a 10 10 b rotational velocity side of the position-velocity diagram along LB (L,B)2.5 × 10 1.3 × 10 3 10 10 the major axis. For a description of the method and a discussion LK (L,B)7.8 × 10 6.4 × 10 4 R in B-band (kpc) 18.6 9.6 3, 5 of the uncertanties involved see Sancisi & Allen (1979). The ro- 25 tational velocity was derived by fitting Gaussian functions with R20 in K-band (kpc) 10.4 7.2 4 Systemic velocity (km s−1) 528 ± 2 1043 ± 46,7 fixed dispersions to the high rotational velocity sides of the line 9 9 Total H I mass ( M)4.1 × 10 1.1 × 10 6, 7 profiles (i.e. the lowest radial velocities on the approaching side H i inclination (◦) ∼>89 ∼90 6, 7 and the highest on the receding one). It was assumed that the 1arcmin= (kpc) 2.76 4.25 7 gas is in circular motion, that there is gas emission at the line of nodes and that the velocity dispersion is constant and equal Notes. (a) More recent determinations of the distances of NGC 891 and to 8 km s−1. Subsequently this rotation curve was used, together NGC 7814 using the tip of the red giant branch are fully consistent with the observed radial H I density profile, as input for the mod- ± +0.7 with these values, i.e. 9.1 0.4Mpcand14.8−0.6 Mpc respectively (b) elling of the data cube. This was done by assuming concentric (Radburn-Smith et al. 2011). Corrected for internal and Milky Way rings. Each ring has its own rotational velocity and gas density. extinction. The inclination angle is fixed to 90◦, determined by modelling References. (1) van der Kruit & Searle (1981); (2) Garcia-Burillo & the H I datacube. The position angle was measured using the to- ◦ Guelin (1995); (3) de Vaucouleurs et al. (1991); (4) Jarrett et al. (2003); tal H I map and the Spitzer data, the values agreed within <0.5 . (5) Abazajian et al. (2004); (6) Oosterloo et al. (2007); (7) this work. There is no evidence for a change of more than 2◦ in position angle and 4◦ inclination out to the outer radius from which the rotation curve is derived (for NGC 891 see also Oosterloo et al. decompositions. We were able to reach new conclusions on the 2007). The effect on the rotation curves of these changes would relative distribution of luminous and dark matter, which differ be less than 1 km s−1. from those mentioned above. The gas density as a function of R was obtained by depro- jecting the total H I maps. The model position-velocity diagram 2. Derivation of the rotation curves was compared (by eye) with the observed one. When necessary, the initial input values of the rotational velocity were changed For NGC 891 we used the H I observations by Oosterloo et al. and the procedure was iterated until a satisfactory matching of (2007) and for NGC 7814 those obtained by Kamphuis (2008). model and observations was reached. This was done indepen- The latter consist of 4 × 12 h integration with the WSRT dently for the two sides of the galaxies. The final adopted ro- (September 2004). The reduction was done with the MIRIAD tation curves are shown plotted on the position-velocity maps

A64, page 2 of 9 F. Fraternali et al.: A tale of two galaxies: light and mass in NGC 891 and NGC 7814

Fig. 2. Upper: position-velocity diagram along the major axis of the Fig. 3. Upper: position-velocity diagram along the major axis of the edge-on galaxy NGC 891. The squares show the rotation curves de- edge-on galaxy NGC 7814. The squares show the rotation curves derived separately for the approaching and the receding side. The con- rived separately for the approaching and the receding side. The contour tour levels are: 0.2, 0.45, 1.0, 2.0, 4.0, 10, 20, 40 mJy beam−1. Lower: levels are: 0.5, 1, 2, 4, 10, 20 mJy beam−1. Lower: position-velocity di- position-velocity diagram along the major axis for the model galaxy. agram along the major axis for the model galaxy. 1 arcmin = 4.25 kpc. The model has been constructed for the symmetric part of the disk. 1arcmin= 2.76 kpc. rotation curve is not well defined due to some beam-smearing and blending with the emission from the inner disk. The sec- (Figs. 2 and 3). The averaged curves are shown in Fig. 4.For ond point shown here is, therefore, rather uncertain. We used each point, the error is the larger between the formal error of the different density distributions for the inner ring and estimate an fit and an uncertainty due to asymmetries between the approach- error of about 12 km s−1. In these central parts, the rotation curve ing and the receding sides. The latter was calculated assuming is slightly asymmetrical: on the receding side it is somewhat that the difference between the two sides corresponds to a 2σ steeper. Further out the approaching and receding rotational ve- deviation (Swaters 1999). locities are the same within <10 km s−1. The rotation curve de- The H I disk of NGC 891 is known to be lopsided (Baldwin rived here differs from that obtained by Sancisi & Allen (1979) et al. 1980). On its southern receding side, it has an extended especially in the inner parts where the higher sensitivity obser- tail (Fig. 2) at a lower radial velocity with respect to the cen- vations by Oosterloo et al. (2007) used here show the inner steep tral part of the disk. However, this decrease in radial velocity rise and the fast-rotating component. The radial extent is about does not necessarily mean a decline in rotational velocity. As the same. pointed out by Sancisi & Allen (1979) it could be due to the lo- The rotation curve of NGC 7814 (Table 3) has a very steep cation of the gas away from the line of nodes. Also, it is possible rise near the centre followed by a slow decline which becomes that non-circular motions dominate in these outer parts where a little more pronounced beyond 2 arcmin (∼8 kpc). The curve the galaxy becomes so asymmetric. We derived, therefore, the in the inner 3 kpc from the centre is not accurately determined rotation curve only for the symmetric part, inside ∼6arcmin because of the poor signal/noise ratio. We estimate an error of (∼17 kpc) from the centre (Table 2). The sharp rise and peak about 15 km s−1. The value of the rotational velocity near the near the centre (R ∼< 1 kpc) indicate the presence of a fast ro- centre (first two points) could be higher than given here. The tating inner H I disk or ring. Although this seems the straight- third point of the rotation curve is well determined (see Fig. 3). forward explanation the possibility of a bar and associated non- There are no large asymmetries; only at large distances from the circular motions cannot be ruled out (Garcia-Burillo & Guelin centre the two sides of the galaxy are not completely symmetri- 1995). However, in the present data there is no indication of cal. Figure 3 shows that there are differences in the H I density non-circular motions. Just outside this central disk or ring the distribution and also in the kinematics. On the approaching side

A64, page 3 of 9 A&A 531, A64 (2011)

260 NGC 891 NGC 7814 240 220 200 180 160 140

Rotation velocity (km/s) 120 100 0 5 10 15 20 0 5 10 15 20 Radius (kpc) Radius (kpc) Fig. 4. The rotation curves of NGC 891 and NGC 7814. The open symbol for NGC 891 outlines some potential contribution from non-circular motions in the centre.

Table 2. Rotation curve of NGC 891. Table 3. Rotation curve of NGC 7814.

Radius Radius vc Radius Radius vc (arcmin) (kpc) ( km s−1) (arcmin) (kpc) ( km s−1) 0.32 0.88 234.5 ± 12.0 0.15 0.64 250.0 ± 15.0 0.80 2.22 191.8 ± 12.0 0.41 1.74 240.0 ± 10.0 1.13 3.11 211.7 ± 6.3 0.67 2.83 230.6 ± 7.3 1.45 4.00 223.3 ± 4.4 0.93 3.96 230.0 ± 6.5 1.77 4.89 222.5 ± 4.0 1.20 5.10 228.1 ± 5.4 2.09 5.78 223.6 ± 3.8 1.47 6.23 227.9 ± 5.2 2.41 6.67 224.2 ± 3.2 1.73 7.36 226.9 ± 5.9 2.73 7.56 226.1 ± 3.4 2.00 8.49 226.1 ± 3.0 3.06 8.44 226.4 ± 3.2 2.27 9.63 222.8 ± 3.9 3.38 9.33 226.6 ± 2.7 2.53 10.76 218.6 ± 2.9 3.70 10.22 226.6 ± 2.7 2.80 11.89 216.2 ± 2.6 4.02 11.11 223.9 ± 2.4 3.07 13.02 214.2 ± 2.6 4.34 12.00 220.1 ± 2.4 3.33 14.16 213.9 ± 3.1 4.66 12.89 217.6 ± 2.3 3.60 15.29 213.9 ± 3.6 4.99 13.78 217.2 ± 2.8 3.87 16.42 213.5 ± 4.5 5.31 14.67 215.6 ± 4.4 4.13 17.55 213.8 ± 5.1 5.63 15.56 210.1 ± 4.5 4.40 18.69 213.6 ± 4.8 5.95 16.44 207.6 ± 4.4 4.67 19.82 214.3 ± 4.9

ff the rotational velocity begins to drop o around 2 arcmin from seems evident. Given the potential uncertainties in the interpreta- the centre whereas on the receding side the decrease seems to ff tion of the first point for NGC 891, we indicate it as an open sym- start a little further out. This rotation curve is di erent from the bol. In order to further investigate the distribution of the mass as flat curve derived by Van der Kruit & Searle (1982). Because / compared to that of the light in the two galaxies we made the of the better signal noise ratio of the new observations it was standard decomposition of the rotation curves in mass compo- possible to derive it also in the inner region (within 1 arcmin nents: bulge, stellar and gaseous disks, and dark matter halo. ∼4 kpc from the centre) where it has a steep rise to a maximum of 250 km s−1 at about 0.6 kpc from the centre followed by a slow − decline to about 214 km s 1 in the outer parts. Its radial extent is 3.1. Photometric data about the same. In order to avoid problems with dust extinction we used data in the 3.6 μm band obtained with the Spitzer Space Telescope 3. Comparison of the rotation curves (Werner et al. 2004). Both NGC 891 and NGC 7814 had been with the distribution of light observed with the Spitzer Telescope and the mosaics were al- ready available in the archive. The observations of NGC 891 The rotation curves for the two galaxies are compared in Fig. 4. had an exposure time of 96.8 s, those of NGC 7814 26.8 s. We They have similar amplitudes but significantly different shapes. converted from the Spitzer units of MJy sr−1 to mag arcsec−2 in The rotation curve of NGC 7814 indicates a more pronounced 3.6 μm band assuming an absolute magnitude for the Sun of mass concentration to the centre than in NGC 891. The corre- M3.6 μm = 3.24 (Oh et al. 2008). We applied the surface bright- spondence with the central concentration of light in NGC 7814 ness correction simply by multiplying our fluxes by 0.91 (Spitzer

A64, page 4 of 9 F. Fraternali et al.: A tale of two galaxies: light and mass in NGC 891 and NGC 7814

3.6μm z-profile (N-W) 3.6μm R-profile (N-E) 14 3.6μm z-profile (S-E) 14 3.6μm R-profile (S-W) )

Bulge + Thin Thick Disks ) Bulge + Thin Thick Disks 2 -2 Disks only

16 16

18 18

20 20 Surface Brightness (mag/arcsec Surface Brightness (mag arcsec 22 22

0 20 40 60 80 100 120 0 50 100 150 200 250 300 350 400 z (arcsec) R (arcsec)

Fig. 5. 3.6 μm photometry for NGC 891. Top panel: distribution of light in the sky in thin (blue) contours overlaid with our best model with bulge and thin and thick disks, see Table 4. Bottom left: light distribution along the vertical direction above and below the plane (circles and triangles) compared with our best model (thick blue curve). Bottom right: light distribution along the plane of the galaxy (circles and triangles) compared with our best model (thick blue curve).

Handbook). We estimated the values of the background in the assumed a de Vaucouleurs profile; leaving the Sérsic index free two images to be 21.06 and 20.70 mag arcsec−2 respectively for gives values very close to 4. In NGC 891 there is a degeneracy NGC 891 and NGC 7814. The 3.6 μm images for the two galax- between the scale-length of the bulge and the scale-height of the ies are shown in Figs. 5 and 6. thick disk and, to obtain a satisfactory result, we preferred to We performed a bulge-disk decomposition of the stellar light fix the latter to a value of hz = 0.8 kpc. Once these values are using GALFIT (Peng et al. 2002, 2010). For both galaxies we ex- fixed, the simultaneous fits of all the other parameters converge tracted the psf directly from the image taking a relatively isolated for both galaxies. star in the field close to the galaxies. The images were masked Figures 5 and 6 show our best photometric models for in order to exclude stars, background galaxies and spurious fea- NGC 891 and NGC 7814 respectively. In both figures the top tures of the CCD. We used a Sérsic profile for the bulge compo- panels show the light distribution (at 3.6 μm) in color shade nent and a perfectly edge-on disk for the stellar disk. NGC 891 and thin (blue) contours from the Spitzer mosaic. The thick could not be fitted with a single disk but it required both a thin (black) contours show the GALFIT solutions obtained using two and a thick disk. The presence of two disks had been found also disks + aSérsic(n = 3) bulge for NGC 891 and one disk + a before in other bands (van der Kruit & Searle 1981; Shaw & r−1/4 bulge for NGC 7814 (see Table 4). The bottom panels in Gilmore 1989; Xilouris et al. 1998). We note that the functional Figs. 5 and 6 show 1D cuts of the data and the models along form of the density distribution in the vertical direction has a the vertical directions (left) and along the plane of the galaxies strong impact on the properties of the disk(s) required by the fit. (right). In the plots perpendicular to the plane we show the data At the moment GALFIT allows only for a sech2 vertical density from both sides (circles and triangles), our best model (thick blue profile, but it is possible that with an exponential profile the two curve) and the contribution of the disk only (thin black curve). components could have very different parameters or the second Note that this latter is completely negligible for NGC 7814. In disk might even not be required. the plots along the plane for NGC 891 one can appreciate that the In fitting the bulge and the disk(s) we first fixed the centres disk in not exponential. This shape has been modelled using the and the position angles of the various components to the same truncation function implemented in GALFIT with rbreak = 120 value. For NGC 7814, we fixed the Sérsic index to 4, i.e. we and Δ rsoft = 280 (see Peng et al. 2010). The truncation is the

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12 12 3.6μm z-profile (N-E) 3.6mu R-profile (S-E) 3.6μm z-profile (S-W) 3.6mu R-profile (N-W)

) Bulge + Disk ) Bulge + Disk -2 14 Disk only -2 14

16 16

18 18

20 20 Surface Brightness (mag arcsec Surface Brightness (mag arcsec

22 22 0 10 20 30 40 50 0 20 40 60 80 100 120 z (arcsec) R (arcsec)

Fig. 6. 3.6 μm photometry for NGC 7814. Top panel: distribution of light in the sky in thin (blue) contours overlaid with our best model with bulge and disk, see Table 4. Bottom left: light distribution along the vertical direction above and below the plane (circles and triangles) compared with our best model (thick blue curve). Bottom right: light distribution along the plane of the galaxy (circles and triangles) compared with our best model (thick blue curve).

Table 4. Spitzer 3.6 μm-band photometric parameters for the two Rmax = 17 kpc and 20 kpc for NGC 891 and NGC 7814 respec- galaxies. tively. The values derived for the NGC 891 disks do not take into account the truncations and are therefore upper limits. As Parameter NGC 891 NGC 7814 expected, the dominant luminosity component in NGC 891 is Thin disk Thick disk that of the disks, about four times brighter than the bulge. On hR (kpc) 4.18 5.13 4.26 the contrary, in NGC 7814 the bulge is totally dominating with a −2 Σe (L,3.6 μm pc ) 443.5 368.9 78.0 bulge-to-disk ratio in the 3.6 μm-band of about 9. In K-band this a hz (kpc) 0.25 0.80 0.44 ratio is about 14 (Wainscoat et al. 1990). 10b 10b 9 Ldisk (L,3.6 μm)4.4 × 10 5.1 × 10 8.4 × 10 Sérsic index 2.99 4.0a re (kpc) 1.80 2.16 3.2. Maximum light (bulge + disk) −2 3 Ie (L,3.6 μm pc ) 525.8 1.12 × 10 q 0.68 0.61 10 10 Figure 7 (upper panels) shows the 3.6 μm-band photometric pro- L (L )2.2 × 10 7.0 × 10 bulge ,3.6 μm files adopted for bulge and disk. For NGC 891 the thin and thick Notes. (a) Fixed in the fit; (b) calculated out to R = 17 kpc without con- disks are combined in a single component with intermediate sidering the truncation. scale-height. The truncation in GALFIT is applied to the pro- jected edge-on profile and it requires deprojection to a face-on view. If the parameter Δ rsoft is small compared to rbreak this can same for both the thin and the thick disk. The disk of NGC 7814 be achieved with good accuracy by simply applying the trunca- does not require any truncation. tion function to the face-on profile (Peng, priv. comm.). However Table 4 gives the photometric parameters of the fits shown in in our case Δrsoft > rbreak and the deprojection is more complex. Figs. 5 and 6. Since our rotation curve fitting routine requires In order to find a functional form for the face-on truncation we a spherical bulge√ we used a spherical-equivalent effective ra- built several mock edge-on truncated disks with GALFIT and dius re, sph = re q (geometrical mean) where q is the axis ra- deproject the light profiles using the “Lucy-method” (Warmels tio. The 3.6 μm-band disk and bulge luminosities in Table 4 are 1988). We then fitted these profiles with exponential disks mul- calculated out to the last measured points of the rotation curves, tiplied by the GALFIT truncation function. We found that good

A64, page 6 of 9 F. Fraternali et al.: A tale of two galaxies: light and mass in NGC 891 and NGC 7814

NGC 891 NGC 7814 14 14 ) 15 Bulge ) 15 2 -2 Bulge 16 16 17 17 18 18 19 Disks 19 20 20 Disk 21 21 SB (mag/arcsec SB (mag arcsec 22 22 ) )

8 2 -2 2 6 /pc pc • • O O 4 1 2 HI (M HI (M 250 250 200 Disks 200 Disk 150 Bulge 150 Bulge

100 100

Rotation velocity (km/s) Gas Rotation velocity (km/s) 50 50 Gas

0 0 0 2 4 6 8 10 12 14 16 18 0 5 10 15 20 Radius (kpc) Radius (kpc)

Fig. 7. Rotation curve decompositions for NGC 891 and NGC 7814. Top panels: surface brightness proﬁles at 3.6 μm for the bulge and the stellar disks built from the parameters of Table 4 (for NGC 891 the two disks have been added). Middle panels: H I surface density. Bottom panels:best ﬁt without a dark matter halo, the M/L ratios of bulge and disk components are given in Table 5. 1 kpc = 21.7 and 33.4 for NGC 891 and NGC 7814 respectively.

results are obtained using for the face-on view the same Δrsoft as Table 5. Fits to the rotation curves. ≈ + 1 Δ for the edge-on view but a Rbreak(face-on) rbreak e rsoft.Theef- fect on the light profile is shown in the upper left panel of Fig. 7. Parameter NGC 891 NGC 7814 The bottom panels of Fig. 7 show the rotation curve de- Maximum light Bulge M/L 1.64 ± 0.07 0.64 ± 0.03 compositions for NGC 891 and NGC 7814 using the above 3.6 μm Disk M/L3.6 μm 0.90 ± 0.02 9.25 ± 0.27 3.6 μm-band photometry. We show here the maximum light 2 a a + Reduced χ 1.05 0.59 (bulge disk) solutions for both galaxies. The fit was obtained Isothermal halo by fitting simultaneously the M/L3.6 μm ratios of bulge and disks Bulge M/L 1.63 ± 0.25 0.71 ± 0.05 ∼ 3.6 μm to the points inside about 3 scale-lengths ( 15 kpc for NGC 891 Disk M/L3.6 μm 0.77 ± 0.16 0.68 ± 0.94 ∼ −3 −3 and 13 kpc for NGC 7814) as it is customary for maximum DM halo ρ0(10 M pc )33.1 ± 16.0 152.4 ± 95.7 light tests (van Albada et al. 1985). The gas surface density (mid- DM core radius r0 (kpc) 1.9 ± 4.42.1 ± 0.6 dle panels) was multiplied by a factor 1.4 to account for Helium. Reduced χ2 1.30 0.39 The fitting parameters are given in Table 5. Notes. (a) Referred to the radial range where the fit is performed (within In the fit for NGC 891 the bulge dominates in the inner parts 3 scale-lengths), see text. and accounts for the steep rise and the inner peak of the rotation curve. The first point of the rotation curve was included in this fit in spite of the uncertainties about its nature and origin pointed which is responsible for most of the light, contributes also the out above. However, the bulge M/L and the quality of the fit largest part of the mass. would not change significantly, first point included or not. At In the NGC 7814 maximum-light decomposition the bulge larger radii, beyond ∼5 kpc, the disk dominates. Its contribution dominates in the inner parts. Bulge and disk together provide to the total mass is about three times as large as that of the bulge. an excellent fit to the rotation curve out to R ∼ 13 kpc. Beyond The shape of the rotation curve is remarkably well reproduced that, a mild discrepancy begins to show up between observed out to the last point. There is no discrepancy between observed and model curve. This is the kind of discrepancy found in the and model curve and no dark matter halo is required here. This, outer parts of spiral galaxies and usually interpreted as evidence however, is not too surprising. The rotation curve is not very for the presence of a dark matter halo. The M/L for the extended: it is not traced beyond the bright optical disk and out 3.6 μm bulge is close to normal, whereas that for the disk is unrealisti- to radii where usually the halo becomes conspicuous. The values cally high. An alternative to such a heavy “dark” disk would be obtained for the M/L ratios of bulge and disk (Table 5) seem a substantial contribution from a dark matter halo (see below). reasonable (cf. Verheijen 1997). The presence of a massive bulge component in this galaxy is In conclusion, for NGC 891 a maximum light (bulge+disk) dictated by the shape of the rotation curve in the inner parts. The solution is quite satisfactory. Bulge and disk together are able to bulge is completely dominant in the inner 5 kpc. In conclusion, explain the rotation curve. Their relative contributions, however, a maximum light solution for NGC 7814 provides an excellent are uncertain given the mentioned degeneracy between bulge fit for most of the rotation curve (out to 13 kpc) but it requires an and thick disk. It is clear, at any rate, that in NGC 891 the disk, unrealistically high M/L3.6 μm for the disk. A dark matter (DM)

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NGC 891 NGC 7814 14 14 ) 15 Bulge ) 15 2 -2 Bulge 16 16 17 17 18 18 19 Disks 19 20 20 Disk 21 21 SB (mag/arcsec SB (mag arcsec 22 22 ) )

8 2 -2 2 6 /pc pc • • O O 4 1 2 HI (M HI (M 250 250 200 Disks 200 Bulge DM halo 150 Bulge 150

100 100 DM halo

Rotation velocity (km/s) Gas Rotation velocity (km/s) Disk 50 50 Gas

0 0 0 2 4 6 8 10 12 14 16 18 0 5 10 15 20 Radius (kpc) Radius (kpc)

Fig. 8. Rotation curve decompositions for NGC 891 and NGC 7814. Top panels: surface brightness profiles at 3.6 μm for the bulge and the stellar disks built from the parameters of Table 4 (for NGC 891 the two disks have been added). Middle panels: H I surface density. Bottom panels:best fit with an isothermal dark matter halo, the parameters of the fits are given in Table 5. 1 kpc = 21.7 and 33.4 for NGC 891 and NGC 7814 respectively. component is, therefore, needed for the region of the disk and to These results clearly indicate that the two main luminous account for the discrepancy in the very outer parts. Clearly, there components in the two galaxies, the disk in NGC 891 and the is for NGC 7814 a disk/halo degeneracy. bulge in NGC 7814, are closely linked to the distributions of mass as traced by the rotation curves. 3.3. Fits with isothermal DM halos 3.4. MOND The maximum light solutions investigated above are useful to understand the possible connection with the light distribution We also compared both rotation curves with the predictions from and the role played by baryons. They show that the distribution the MOdified Newton Dynamics (MOND) (Milgrom 1983). The of light and that of mass are very similar inside each of the two rotation curve of NGC 891 is reproduced, using the standard 2 −2 −1 galaxies. Here we add dark matter halos to the fits and model value of a0 = 3700 km s kpc with best-fit M/L ratios for the DM distribution as a standard isothermal sphere (van Albada the bulge and disk components of 2.0 and 0.5 respectively. The et al. 1985). We fit the four parameters (M/L ratios and halo pa- result is shown in the upper panel of Fig. 9. The quality of this rameters) simultaneously. The resulting fits are shown in Fig. 8 fit is however not very good leading to a reduced χ2 of 4.8. and the values of the parameters are given in Table 5.Usinga For NGC 7814 a MOND fit with free M/L3.6 μm ratios for NFW profile (Navarro et al. 1997) would not change signifi- the bulge and disk components requires for the latter a very high cantly our results. The best-fit values of the concentration pa- value of 4.6. This is needed to fit the outer parts of the rota- rameters are above 10 for both galaxies. If we fix them to c = 10 tion curve. In this case the M/L3.6 μm of the bulge is 0.77. If the we obtain M/L ratios comparable to those in Table 5, except for M/L3.6 μm of the disk is kept fixed to a more acceptable value the M/L of the disk in the free fit of NGC 7814, which becomes of 1, the MOND fit using the standard interpolation function of 2 M/L3.6 μm = 2.7; if however we fix this latter to 0.7 the fit is still Milgrom (1983) is not acceptable (χ = 7.2). The bottom panel acceptable. of Fig. 9 shows the model predictions for this function (thin In NGC 891 the results are very similar to those obtained curve) and also for the so-called “simple” function of Famaey & above with the maximum light solution. The fit is still excel- Binney (2005). Clearly this second function gives a much better 2 lent. The values for the M/L3.6 μm of bulge and disk differ only fit (χ = 1.3 bulge M/L3.6 μm = 0.83). We also used the “simple” slightly and the dark halo plays a minor role. The exclusion of function for NGC 891 but we did not find a significant improve- the first point of the rotation curve in the fit would not make any ment of the fit (disk M/L3.6 μm = 0.23, bulge M/L3.6 μm = 1.9, difference. χ2 = 4.1). In order to make the fit with the standard interpola- NGC 7814 is more puzzling than NGC 891 because of the tion function compatible with the data of NGC 7814, the dis- disk/halo degeneracy. However, a fit with all four parameters tance to this galaxy should be increased by a factor ∼1.5. This (M/L ratios and halo parameters) free converges and gives ac- is not compatible with the new error determination that allows, ceptable results. The bulge is still dominant in the inner 7−8 kpc at 3σ, an increase of at most 15% (Radburn-Smith et al. 2011). and very close to maximum with only a minimal change in To improve significatly the fit of NGC 891’s rotation curve the M/L3.6 μm. Further out, the halo now completely dominates in distance should be decreased by a factor >2, incompatible with the place of the disk. the new determinations.

A64, page 8 of 9 F. Fraternali et al.: A tale of two galaxies: light and mass in NGC 891 and NGC 7814

NGC 891 - MOND We conclude that in both galaxies, in their bright optical parts, the distribution of mass seems to follow closely the dis- 250 tribution of light. This implies that either the baryons dominate or the dark matter is closely coupled to the luminous component. 200 Bulge It would be interesting to repeat the same study on galaxies with extreme morphologies such as NGC 7814 and NGC 891 but seen 150 at lower inclination angles. Disks

100 Acknowledgements. WethankT.S.vanAlbada,P.C.vanderKruit,T. Oosterloo, R. Peletier, R. Sanders, M. Verheijen, and E. Xilouris for helpful comments and stimulating discussions. We thank C. Peng for advice about Rotation velocity (km/s) Gas 50 the ﬁtting of edge-on disks with GALFIT and D. Radburn-Smith for providing the new distances of NGC 891 and NGC 7814. F.F. is supported by the PRIN-MIUR 2008SPTACC. P.K. is supported by the Alexander von Humboldt 0 Foundation. The Westerbork Synthesis Radio Telescope is operated by ASTRON 0 2 4 6 8 10 12 14 16 18 (Netherlands Institute for Radio Astronomy) with support from the Netherlands Radius (kpc) Foundation for Scientiﬁc Research (NWO). This work is based in part on ob- NGC 7814 - MOND servations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with Standard NASA. We are grateful to David W. Hogg, Michael R. Blanton, and the Sloan 300 FB Digital Sky Survey Collaboration for the gri mosaics of NGC 7814 and to C. Marmo (TERAPIX) for the WIRCam/CFHT multi-color image of NGC 891. 250 TERAPIX is funded by the French national research agency (CNRS/INSU), the Programme National de Cosmologie (PNC), the Service d’Astrophysique of 200 the Commissariat l’Energie Atomique (CEA/SAp), the Institut d’Astrophysique de Paris (IAP), and the European FP5 RTD contracts “Astrowise” and “AVO” Bulge 150 (Astrophysical Virtual Observatory).

100 Rotation velocity (km/s) Disk References 50 Gas Abazajian, K., Adelman-McCarthy, J. K., Agüeros, M. A., et al. 2004, AJ, 128, 0 502 0 5 10 15 20 Baldwin, J., Lynden-Bell, D., & Sancisi, R. 1980, MNRAS, 193, 313 Radius (kpc) de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H. G., et al. 1991, Third Reference Catalog of Bright Galaxies (Springer-Verlag), Vols. 1–3, XII (RC3) Fig. 9. Comparison between the rotation curves of NGC 891 and Famaey, B., & Binney, J. 2005, MNRAS, 363, 603 NGC 7814 and the MOND predictions. For NGC 7814 we show the Garcia-Burillo, S., & Guelin, M. 1995, A&A, 299, 657 predictions obtained using the standard interpolation function (thin blue Jarrett, T. H., Chester, T., Cutri, R., Schneider, S. E., & Huchra, J. P. 2003, AJ, curve) and the so-called “simple” (FB) function of Famaey & Binney 125, 525 (2005) (thick blue curve). Kalnajs, A. J. 1983, in Internal Kinematics of Galaxies, ed. E. Athanassoula (Dordrecht: Reidel), IAU Symp., 100, 87 Kamphuis, P. 2008, Ph.D. Thesis, University of Groningen Kent, S. M. 1986, AJ, 91, 1301 4. Conclusions Oh, S.-H., de Blok, W. J. G., Walter, F., Brinks, E., & Kennicutt, R. C. 2008, AJ, The two galaxies NGC 891 and NGC 7814 are represen- 136, 2761 Oosterloo, T., Fraternali, F., & Sancisi, R. 2007, AJ, 134, 1019 tative of two extreme morphologies, with the disk dominat- Milgrom, M. 1983, ApJ, 270, 365 ing in NGC 891 and the bulge almost entirely dominating in Navarro, J. F., Frenk, C. S., & White, S. D. M. 1997, ApJ, 490, 493 NGC 7814. We have derived new rotation curves for both. Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H.-W. 2002, AJ, 124, 266 Contrary to previous reports (van der Kruit & Searle 1982; Peng, C. Y., Ho, L. C., Impey, C. D., & Rix, H.-W. 2010, AJ, 139, 2097 Radburn-Smith, D., et al. 2011, ApJS, submitted van der Kruit 1983), the shapes of these curves are found to be Sancisi, R. 2004, in Dark Matter in Galaxies, ed. S. Ryder, D. Pisano, M. Walker, significantly different. They indicate that in NGC 7814 the mass & K. Freeman, IAU Symp., 220, 233 is more concentrated to the centre as compared to NGC 891. It Sancisi, R., & Allen, R. J. 1979, A&A, 74, 73 resembles, therefore, the distribution of the luminosity, which Shaw, M. A., & Gilmore, G. 1989, MNRAS, 237, 903 is more centrally concentrated in NGC 7814 (bulge) than in Swaters, R. A. 1999, Ph.D. Thesis, University of Groningen van Albada, T. S., & Sancisi, R. 1986, Phil. Trans. R. Soc. London, Ser. A, 320, NGC 891 (disk). 447 A decomposition in bulge, disk and halo shows that in van Albada, T. S., Bahcall, J. N., Begeman, K., & Sancisi, R. 1985, ApJ, 295, NGC 891 the disk is the major mass component. The bulge 305 contributes about one fourth of the total dynamical mass. In van der Kruit, P. C. 1995, in Stellar Populations, ed. P. C. van der Kruit, & G. Gilmore (Dordrecht: Kluwer Acad. Publ.), IAU Symp., 164, 205 NGC 7814 the bulge dominates almost entirely the total lumi- van der Kruit, P. 1983, PASAu, 5, 136 nosity (90 percent of total). In the distribution of mass it is un- van der Kruit, P. C. 1999, Astrophys. Space Sci., 267, 227 doubtedly the dominant component in the inner parts. In the van der Kruit, P. C., & Searle, L. 1981, A&A, 95, 116 outer parts the dark matter halo takes over. The disk, unless it van der Kruit, P. C., & Searle, L. 1982, A&A, 110, 79 has an unrealistically high M/L ratio, seems to be a minor com- Verheijen, M. 1997, Ph.D. Thesis, Groningen University Wainscoat, R. J., Hyland, A. R., & Freeman, K. C. 1990, ApJ, 348, 85 ponent. Warmels, R. H. 1988, A&AS, 72, 427 Standard MOND fits do not work perfectly for both galaxies. Werner, M. W., Roellig, T. L., Low, F. J., et al. 2004, ApJS, 154, 1 NGC 7814 is well fitted by the “simple” interpolation function Xilouris, E. M., Alton, P. B., Davies, J. I., et al. 1998, A&A, 331, 894 (Famaey & Binney 2005).

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